They don't accept my modem event log as evidence of a problem
Try to document packet loss along with the T3 errors in the log. I think they(Tier 2) should be able to ping your modem for packet loss/latency. Try a program like Pingplotter to plot and trace packet loss over time. If you can equate packet loss with the T3 errors at the same time they are pinging your modem, they should see a problem. The problem is Tier 1 is forced to follow a script that very quickly boxes them into a corner troubleshooting wise.
Also, can you post your signal levels? Did you get the upgrade to a OFDMA upstream channel? Did you notice these problems around the time you got it?